Electromagnetic interference ("EMI") is the electromagnetic noise which radiates from operation of electrical and electronic devices. The emanation of such noise can radiate into space, or along electrical power lines, or both. The portion of the EMI in the radio frequency ("RF") range may cause harmful interference with radio communications.
A computing device can represent a significant source of EMI in a communications environment. The modern design trend of computing devices is with plastic housings which, unless shielded, are relatively transmissive of EMI. In order to shield the exterior environment of such a computing device, it is necessary to apply a metallic coating to the inner surfaces of the plastic housing. The Federal Communications Commission has regulated the emission of RF energy from computing devices in 47 C.F.R. .sctn. 15.801 et seq. The manufacturers of such computing devices are now generally required to verify compliance of the devices with the field strength limitations set forth in the regulations.
One conventional method of coating plastic electronic cabinets with EMI shielding has included the steps of sand or grit blasting the cabinet interior with abrasive material, cleaning the cabinet to remove abrasives and other impurities, and spraying metal film (usually zinc) by an arc or plasma spraying process. However, if any of the surface area of the cabinet interior is missed by the sandblasting operation, the metal adhesion is very poor since it relies on the abraded surface for adhesion.
Another conventional method is to subject the cabinet to chemical etching, such as by a chromic acid rinse. This method generally results in all surfaces of the cabinet, interior and exterior, being etched. Subsequently, a metallic coating can then be applied to the interior surfaces for example by an electroless metallic coating process.
The prior art methods hold certain disadvantages reflected in relatively slow production rates, need for special equipment, environmental control requirements for containing abrasive grit or disposing of hazardous chemical wastes resulting from chemical etching and the like. Of particular concern is the adhesion of the metallic film to the plastic cabinet. Any interruption in surface preparation can result in poor adhesion and undesired emission of EMI. In fact, adhesion requirements have now been made the subject of equipment manufacturer specifications, and an exemplary specification requires adhesion of the metallic film to withstand a pull force of approximately 70 oz/in.